The use of ultrasound waves for distance measurement is well known in the art. Basically, an ultrasonic wave is transmitted from a fixed unit, to a sensor associated with an object to which the distance is to be measured. Alternatively, the wave may be transmitted from the object and received at the fixed unit. The propagation time of the ultrasonic wave is used to determine the distance between the fixed unit and the object, according to the known sound velocity in the object's environment. Such a system is described, for example, in U.S. Pat. No. 4,751,689 which is incorporated herein by reference. In the 4,751,689 patent, radio frequency waves are used to communicate between the fixed unit and the sensor in order to simultaneously initiate the propagation-time measurement with the wave transmission.
Such distance measurement is further used for position determination. Generally, three fixed units at known fixed points determine simultaneously, or within a short interval, the distance to the object. The position of the object in three dimensions is determined accordingly. Such systems are described for example in U.S. Pat. Nos. 3,731,273, 4,862,152 and 5,142,506, which are incorporated herein by reference. In order to continuously track an object, multiple position determining cycles are performed consecutively. The rate at which the cycles are repeated is called the refresh rate and is generally desired to be as high as possible in order to provide quasi-continuous tracking.
In order to track both position and orientation of an object, such as a helmet, it is desirable to attach at least three sensors to the object. U.S. Pat. No. 4,807,202 describes a visual environment simulator which tracks the spatial coordinates and orientation of a viewer. Three transmitters associated with the viewer send intermittent signals to three receivers in order to provide the information for tracking.
One of the problems with ultrasound tracking is the slow propagation velocity of ultrasound waves and therefore the low refresh rate. In the simulator of the 4,807,202 patent, the refresh rate is even lower because of the need to transmit consecutively three ultrasonic waves, in every cycle. In addition, the positions of all three transmitters are not determined simultaneously.
U.S. Pat. Nos. 5,339,259 and 5,495,427, which are incorporated herein by reference, enhance the refresh rate of tracking systems by measuring the distance between the sensors and fixed units based on a phase shift of transmitted ultrasonic waves, rather than according to propagation time. The 5,339,259 patent suggests using a plurality of distinct frequencies in order to allow simultaneous measurement of nine distances. The systems of the 5,339,259 and 5,495,427 patents are oriented to tracking position and orientation of only one object, which has a wire connection to the tracking system. The system of the 5,495,427 patent is not capable of tracking more than one object, since it determines the distance of the object from the fixed unit based on only one sensor. The positions of the other sensors are determined relative to this sensor. The 5,339,259 patent requires an additional complete tracking system for each further object to be tracked, since the tracking system is specifically planned for tracking one object.
Another tracking system is described in U.S. Pat. No. 5,412,619, which is incorporated herein by reference. In this system the refresh rate is enhanced by consecutively transmitting waves of a plurality of different frequencies toward the tracked object. Thus, it is possible to transmit another wave before the previous wave has arrived at the target. The 5,412,619 patent also suggests using an extra, fourth, fixed unit to overcome momentary obstructions in the path of the ultrasound waves, and provide extra data for discovering obvious errors.
The above-mentioned tracking systems have a limited area in which the objects may be tracked. In most of these systems, the refresh rate is dependent on the diameter of the area in which the object is tracked. The larger the area, the lower the refresh rate, since the waves need to propagate longer distances. In order to prevent tracking errors, it is generally not possible to transmit another wave of the same frequency before a previous wave has propagated the full length of the tracked area.
U.S. Pat. No. 4,333,170, which is incorporated herein by reference, describes a passive ultrasonic detection system which detects the direction to an ultrasound emitting object. The system uses a plurality of receivers to determine the direction to the object. The system identifies from which object each receiver received signals according to the frequency of the received signals. However, this system does not provide the distance to the object or the object's orientation, and since it has no indication of the time of transmission of the signals could not determine the distance in any known manner.
U.S. Pat. No. 5,528,232 describes a communication system which is used for locating objects within a warehouse. A travel distance, defined as the distance between a locator and the object along unobstructed paths, is determined according to the propagation time of an ultrasonic wave. The 5,528,232 patent suggests achieving greater location precision by using a greater number of locators. This system is slow and cannot determine simultaneously the positions of more than one object. Also, this system does not provide true position information, but rather unobstructed-path information, and therefore is not suitable for high refresh-rate position and orientation tracking.